Digital Particle Image Velocimetry Technique Research Articles

Validation of CFD and Thermal Hydraulics Codes by Digital Particle Image Velocimetry

Abstract Experimental validation of thermal hydraulics and CFD codes of scaled down models and full models of Nuclear Reactor components is very important for reactor system designers and reactor safety experts for optimal design and providing best safety features of the reactor systems. Over the period of time several techniques have been developed for this purpose. Among them Digital Part icle Image Velocimetry (DPIV), a laser based optical technique, is the recent, innovative, promising and flexible enough one so that it can be adapted to a large variety flows. By this technique test time is reduced significantly. DPIV is a non-intrusive optical measurement technique, which allows capturing several thousand velocity vectors within large flow fields instantaneously. DPIV technique is being used in very wide and different applications, from micro flows over combustion to supersonic flows for both industrial needs and research. 2-D DPIV facility having non-intrusive, instantaneous flow visualization with high spatial and temporal resolution features has been developed. It is used for measurement of flow velocity vectors at many (e.g. thousands) points in a flow field simultaneously and it provides instantaneous vector fields and displays velocity vectors in rea l-t ime. DPIV combines flow visualization with accurate quantitative velocity data. 2-D Velocity vectors are obtained by analysis of displacement of tracer particles on images in a known time. The development of visualizat ion tools are powerful enough to at tack the complexity of a resulting flow field but also flexible enough so that they can be adapted to a large variety of different flow. DPIV system has Laser light source, Laser beam delivery system, High speed camera interface, DPIV image acquisition & analysis software, CCD camera, computer controlled synchronizer, seeding particles & PC. This system will enhance our capability of thermal hydraulics, neutronics and mult iphysics modelling by experimental support & actual measurement of parameters, in addition to codes we had with us. Application of DPIV is must for the flow in the separation and reattachment regions where it is directly related to drag and heat transfer coefficients and for Vortex shedding. This facility is being used for basic research of code verification, code development, design optimizations and safety analysis.

Two-phase interactions through turbulent events as described by fluid–particle correlations

An experimental investigation of a vertical upward, two-phase pipe flow was undertaken to measure kinematic parameters of the fluid and solid phases. The kinematic parameters included Reynolds stress distributions based on quadrant analyses that provided insight in understanding the behavior of two-phase kinematic correlation profiles. The data collected was based on a two-color digital particle image velocimetry (DPIV) technique that simultaneously measured the velocity fields of the fluid and solid phases. From quadrant analysis results, differences in Reynolds stress quadrant profiles between the single- and two-phase conditions were observed near the wall in the range 0.8> r/R>0.55, corresponding to wall distances between 35 and 75 viscous lengths ( y+). Correlation coefficients between the two phases were then calculated, using the fluctuating velocity components of each phase. The extent of the interaction between the two phases was tracked by the changing correlation values versus distance from the wall. The correlation of the fluid and solid phase velocities was highest in the core region of the pipe ( y+∼120), where the effect of turbulent events is reduced; low correlation coefficient values were found at y+<75, where differences of magnitudes, inflection points, etc. of burst and sweep event quadrant analysis profiles were observed. The extent of the influence of wall dynamic turbulent events on the solid phase was observed both by the differences in the relative Reynolds stress quadrant profiles and, more readily, by the changing values of two-phase axial and radial correlation coefficients determined from the simultaneous fluid and solid fluctuating velocities measured by the two-color DPIV methodology. These changing values of the correlation coefficients across the tube reflect the different responses of low inertia (fluid tracers) and high inertia (solid phase glass spheres) particles to the turbulent events. Similar profiles of the axial and radial correlation coefficients were observed, indicating that for the geometry and flow conditions considered, one velocity component of each phase was sufficient to track the spatial extent of turbulent event effects and their interactions with the fluid and solid phases. It is found that the two-color DPIV methodology and two-phase correlation results can give critical insight into the performance of thermal-fluid processes, as burst and sweep events have a large impact on the kinematics and dynamics of particles in the two-phase flow.

Dense-phase velocity fluctuation in a 2-D fluidized bed

Abstract This work presents an investigation of the perturbations induced by the bubbles in a 2-D fluidized bed. A combination of Digital Image Analysis (DIA) and Particle Image Velocimetry (PIV) techniques was developed to characterize the dense and bubble phases. The analysis of the mean and the instantaneous fluctuations of the velocity of the dense phase, together with the solid movement around bubbles, allowed for the measurement of the influence region, distinguishing an upward moving dense phase in the nose and the wake of the bubble (drift) and a downward moving dense phase in the sides of the bubble. For an isolated bubble, we measured the drift area within the total influence region and related the size of these regions to the equivalent diameter of the bubble. This work also presents results on the volumetric dissipation of kinetic energy, where we concluded that the energy dissipation in the dense phase is proportional to the square of the bubble velocity.

Empirical description of flow parameters in eccentric flow inside a silo model

The paper presents the methodology of empirical description and statistical analysis of velocity profiles that were depicted by the Digital Particle Image Velocimetry technique (DPIV). Experimental runs were recorded by the high resolution camera in the model with vertical walls. Here we analyze the eccentric discharge with the outlet located in the bottom close to the right vertical wall of the model. On the base of the experimental results we present an empirical analysis of velocities and calculation of the flow rate in two proposed descriptions of the flow. Velocity functions were presented by the exponential function (the modified Gauss type), by the multiple regression and by the ch function. Also the flow rate was calculated for two presented descriptions. Empirical calculations of the stagnant zone boundary was also presented using the readings from velocity profiles.

Buoyancy Dominated He–O2 Separated Jet Mixing in a Tubular Reactor

Mixing of two variable-density jets, one of which is essentially a negatively buoyant jet (He) and the other a nonbuoyant jet (O2) is studied experimentally using digital particle image velocimetry technique for separated nozzle geometry. Two jets are separated from each other, i.e., He jet exit plane is located upstream of the O2 jet exit plane. Experiments were carried out keeping the nonbuoyant O2 jet at constant Reynolds number (Re2=245) and varying the Richardson number (Ri1=1.8,4.7,16.4) of the buoyant He jet. The interaction between two jets as a function of buoyancy strength of helium jet is investigated. The flow visualization images clearly demonstrate the growth and shape of buoyant jet as a function of Richardson number. Mean velocity and vorticity field results provide quantitative picture about the mixing and interaction between the two jets. The stream trace results show the flow structures, i.e., recirculation zone and foci of vortex structures as a function of Richardson number. The mixing between two jets takes place at far downstream region for low Richardson number (Ri1=1.8). At high Richardson number (Ri1=16.4), the buoyant He jet is located near its exit plane without any direct interaction with the O2 jet. At the intermediate Richardson number (Ri1=4.7), the buoyant jet encroaches the nonbuoyant O2 jet at a favorable penetration distance and a good amount of mixing between the two jets take place. The jet growth results based on the y0.5-location development in the streamwise direction clearly demonstrate the interaction between the He and O2 jets in the interjet region. The entrainment coefficient, vorticity magnitude, and turbulent kinetic energy magnitude are maximum at the intermediate Richardson number (Ri1=4.7) demonstrating direct relationship among each other. The mixing between buoyant He jet and nonbuoyant O2 jet is a function of both shear between the two jets and the strength of buoyant plume.

Digital particle image velocimetry studies of nasal airflow

Understanding the properties of airflow in the nasal cavity is essential to understanding physiologic and pathologic aspects of nasal breathing. Many attempts have been made to evaluate nasal airflow patterns using the best possible analytical methods available at the time. Recently, digital particle image velocimetry (DPIV) and computational fluid dynamic methods have been applied to this area. Digital PIV is an experimental method used to evaluate airflow in an accurately reproduced transparent model of the nasal cavity. In this review, use of the DPIV procedure in the study of nasal airflow, airflow patterns in quiet respiration, and changes to airflow after modification of the nasal turbinates are reviewed, along with aspects of the DPIV technique and the future role of DPIV in this field of research.

Flow Control of Annular Compressor Cascade by Synthetic Jets

An experimental investigation conducted in a stationary annular cascade wind tunnel demonstrated that unsteady flow control using synthetic jets (zero mass flux) could effectively reduce flow separation in the axial compressor cascade. The synthetic jets driven by speaker were introduced through the casing radially into the flow-field just adjacent to the leading edge of the compressor cascade. The experimental results revealed that the aerodynamic performance of the compressor cascade could be improved amazingly by synthetic jets and the maximum relative reduction of loss coefficient was up to 27.5%. The optimal analysis of the excitation frequency, excitation location was investigated at different incidences. In order to obtain detailed information on flow-field structure, the digital particle image velocimetry (DPIV) technique was adopted. The experimental results indicated that the intensity of wake vortices became much weaker and streamlines became smoother and more uniform with synthetic jets.

Near-field developments of elliptic jets in crossflow

An experimental study has been carried out for aspect ratio AR = 0.3 and 3 elliptic jets in crossflow, with the results being compared to a circular jet in crossflow using the digital particle image velocimetry technique. Results show that for AR = 0.3 and 3 elliptic jets, which are significantly streamlined and blunt with respect to the crossflow respectively, unsteady foci observed earlier in AR = 0.5 and 2 elliptic and circular jet lee-side regions are either non-existent or not well defined. Mean peak vorticity values along leading-edge and lee-side shear layers are also observed to be higher for elliptic jets than for the circular jet, regardless of aspect ratio. Intriguingly, AR = 0.3 elliptic jet had lower vorticity values than its AR = 0.5 counterpart along the shear layers and could be due to rapid vortex entanglements of the leading-edge vortices within short distances away from the AR = 0.3 elliptic jet exit. Velocity plots also show ‘bifurcations’ for the transverse velocity component and demonstrated reduced activities associated with the counter-rotating vortex pair formation as the aspect ratio increases. Analysis of the velocity profile developments downstream of the jets shows multiple velocity maxima and minima associated with various features of the jet in a crossflow phenomenon and that crossflow recovery at the lee-side regions will be slower for high aspect ratio jets. Lastly, results also show that although stronger flow reversals occur for high aspect ratio jets, they do not lead to more intense counter-rotating vortex pair formations over the low aspect ratio elliptic jets.

The influence of wavy walls on the transport of a passive scalar in turbulent flows

We carry out an experimental study to investigate the turbulent transport of a passive scalar in turbulent flows between a flat top wall and two different complex surfaces and describe the influence of the different surface geometries on the mixing properties.In order to achieve a homogeneous and inhomogeneous reference flow situation two different types of surface geometries are considered: a sinusoidal bottom wall profile and a profile consisting of two superimposed sinusoidal waves.The laboratory investigations were performed by the use of a combined digital particle image velocimetry (DPIV) and planar laser-induced fluorescence (PLIF) technique to examine the spatial variation of the streamwise, spanwise and wall-normal velocity components and to assess the concentration field of the scalar. Statistically sufficient image sequences (frame rate 4 Hz) are obtained from both DPIV and PLIF to calculate time-averaged statistics of the velocity field and the scalar field. We discuss the influence of the complex surfaces on large-scale structures present in the flow field and the transport of the passive scalar. A higher spanwise spreading of the scalar plume is observed compared to plane channel flows. Due to the influence of the complex surfaces a higher transport normal to the mean flow direction is found. The mixing properties of the turbulent flow are enhanced for the surface composed of two superimposed waves compared to the sinusoidal wavy surface. This is consistent with the scaling of large-scale structures found for this surface geometry.

Air Flow Structure Over Short-gravity Breaking Water Waves

Despite its importance for momentum and mass transfer across the air-sea interface, the dynamic of the airflow over breaking waves is largely unknown. To fill this gap, velocity and vorticity distributions above short-gravity breaking waves have been measured in a wind-wave tank.A Digital Particle Image velocimetry technique was developed to accomplish these measurements above single breaking waves, propagating in mechanically-generated wave groups and forced by the wind. By varying the wind speed and initial characteristics of the groups, the airflow structure was captured over waves at different stages of the breaking process, and breaking with various intensities.The instantaneous airflow separated from a sharp breaking crest is very similar to that occurs over the backward facing step. The separation bubbles dynamic is however strongly unsteady: the steeper the wave crest and the larger the Reynolds number based on the crest-height, the higher the separated-layer and the farther downwind the reattachment point. Instantaneous flow topology displays specific features of three-dimensional separation patterns. The tangential stress above the wave profile do not exhibits spikes at reattachment but grows progressively downwind from zero at reattachment to a value at the next crest approximately that found at the upwind breaking crest. Static pressure measurements revealed that large pressure drops are generated by vortices in the separated layer, as found in separated flows over solids.This study may provide useful data for theoretical and numerical modeling of the flow and associated phenomena.

Experimental investigation of mixed convective flow over a wavy wall

An experimental study is carried out to address mixed convection from a heated wavy surface. The channel flow between the sinusoidal surface and a flat top wall is investigated by means of a combined digital particle image velocimetry (DPIV) and planar laser-induced fluorescence (PLIF) technique to examine the spatial variation of the streamwise and wall-normal velocity components, and to assess the concentration field of a tracer dye injected into the fluid. We discuss the influence of mixed convection on turbulence quantities and scalar transport properties. Due to the influence of mixed convection we find asymmetric mean velocity profiles and increased momentum transport in the vicinity of the heated surface. The transport of the tracer dye is characterized by enhanced vertical transport due to buoyancy effects and enhanced spanwise transport due to the presence of longitudinal flow structures induced by the mixed convection. We identify two dominant scalar structures with the most influence on vertical transport. The first one is induced by buoyancy effects, the second one by a combination of buoyancy effects and local wall curvature. Thus the transport properties are additionally enhanced compared to mixed convection from a flat plate by the presence of the wavy surface.

Wake-Structure Formation of a Heaving Two-Dimensional Elliptic Airfoil

This paper is prompted by a recent numerical study that shows that for a two-dimensional (2-D) elliptic airfoil undergoing prescribed heaving motion in a viscous fluid, both leading-edge vortices and trailing-edge vortices contributed to the formation of the wake structures. However, an earlier dye-visualization study on a heaving NACA 0012 airfoil appears to show that the wake structures were derived from trailing-edge vortices only. The dissimilarity in the two studies remains unclear because there is no corresponding experimental data on a 2-D heaving elliptic airfoil. In this study, digital particle image velocimetry technique was used to investigate the wake-structure formation of a 2-D elliptic airfoil undergoing simple harmonic heaving motion. For the range of flow conditions investigated here, our results show that the type of wake structures produced is controlled by when and how the leading-edge vortices interact with the trailing-edge vortices

Laminar Flame Speeds of Preheated iso-Octane/O2/N2 and n-Heptane/O2/N2 Mixtures

Laminar flame speed measurements are carried out for premixed i so-octane/air and n-heptane/air mixtures under conditions of atmospheric pressure, equivalence ratios ranging from 0.7 to 1.4, and unburned mixture temperatures of 298, 360, 400, and 470 K using the counterflow flame technique. These experiments employ the digital particle image velocimetry technique to characterize the two-dimensional flow field upstream of the flame. As such, the reference stretch-affected flame speed and the imposed stretch rate can be simultaneously determined. By systematically varying the imposed stretch rate, the corresponding laminar flame speed is obtained by linearly extrapolating to zero stretch rate. In addition, the effect of nitrogen dilution level on the laminar flame speed is investigated by varying the nitrogen molar percentage in the oxidizer mixture from 78.5 to 80.5%. These results are further used for the determination of overall activation energies at different equivalence ratios. The experimental laminar flame speeds are subsequently compared with the computed values using two iso-octane reaction mechanisms and two n-heptane reaction mechanisms available in the literature, followed by discussion and sensitivity analysis.

Intraoperative brain ultrasound: A new approach to study flow dynamics in intracranial aneurysms

The aim was to evaluate the potential of contrast-enhanced ultrasound to visualize the hemodynamics in intracranial aneurysms during neurosurgical intervention and to quantify the ultrasound data using digital particle image velocimetry (DPIV) technique. Aneurysms were scanned through the intact dura mater, preclipping and again postclipping after closure of the dura. After intravenous injection of Optison™, angio-like views of the vascular tree surrounding the aneurysm, including the aneurysm sac, were obtained. Single ultrasound contrast agent microbubbles could be visualized in the aneurysm sac and the flow dynamics could be assessed in vivo. Spatial and temporal distributions of the velocity in the aneurysm and in the parent vessels were measured with DPIV using the backscattered signals from the microbubbles. Subsequently, the fluid stresses, vorticity, circulation, etc., were calculated from the velocity fields. We demonstrate in this paper that intraoperative contrast-enhanced ultrasound can be used to quantify the flow dynamics within an aneurysm. (E-mail: thoelscher@ucsd.edu)

Two-frame 3D particle tracking

A whole-field three-dimensional (3D) particle tracking velocimetry (PTV) tool for diagnostics in fluid mechanics is presented. Specifically, it is demonstrated why and when PTV is the natural choice in 3D applications compared to particle image velocimetry (PIV). Three different tracking methods are investigated, namely the nearest neighbour, the neural network and the relaxation method. In order to demonstrate the use of PTV for 3D applications, the selected tracking schemes are implemented for use with the defocusing digital particle image velocimetry (DDPIV) technique. The performance of the tracking algorithms is evaluated based on synthetic 3D information. Furthermore, the potential benefit of a merging between the PIV and PTV approaches is explored within the DDPIV framework. The results show that the relaxation tracking method is the most robust and efficient, while the combined PIV/PTV analysis brings significant improvements solely with the neural network scheme. In terms of errors, PTV is found to be more sensitive to particle reconstruction errors than the DDPIV cross-correlation analysis.

Experiments and theory for two grids turbulence

Turbulence generated by a pair of oscillating in a water tank is investigated using the digital particle image velocimetry technique. Vertical and horizontal components of fluctuating velocities are measured. Experimental profiles of the turbulent kinetic energy k, and the energy dissipation rate, x, obtained from the fluctuating velocities, are presented and compared with theoretical predictions. Other properties of interest, such as the length scale and the turbulent viscosity, obtained in the present study, are also reported. The turbulence generated with the pair of may be classified as isotropic and theoretically the flow may be analyzed using the k-e model. The experimental data agree well with the theoretical predictions. Also data of different sources found in literature, for pairs of vibrating grids, are compared with the present experimental results and theoretical predictions, showing good agreement. The studies conducted at the Laboratory of Environmental Hydraulics from the Department of Hydraulics and Sanitary Engineering, EESC/USP, permits to affirm that double oscillating grids turbulence can be viewed as nearly isotropic.

Effects of jet velocity profiles on a round jet in cross-flow

This paper reports the results of an experimental investigation on the effects of jet velocity profiles on the flow field of a round jet in cross-flow (JICF) using laser-induced fluorescence and digital particle-image velocimetry techniques (DPIV). Tophat and parabolic jets were considered, with the momentum ratios (MRs) ranging from 2.3 to 5.8. Results show that the thicker shear layer associated with a parabolic JICF is able to delay the formation of leading-edge and lee-side vortices when compared to the tophat JICF at the corresponding MR. As a result, there is an increase in jet penetration and a reduction in the near-field entrainment of cross-flow fluid by a parabolic JICF. Also, the less coherent nature of the leading-edge and lee-side vortices in a parabolic JICF is more likely to break up sporadically into smaller-scaled vortices. In addition, DPIV results show that a parabolic JICF exhibits not only a faster velocity recovery of cross-flow fluid at the jet lee-side than the corresponding tophat JICF, it also consistently registers a higher magnitude of the peak average vorticity than the tophat JICF for all MR considered. Despite these differences, the time-averaged flow topology for both cases share many salient features.

Simultaneous velocimetry/accelerometry measurements in a turbulent two-phase pipe flow

A two-color digital particle image velocimetry and accelerometry (DPIV and DPIA) measurement technique is described that records the velocity and acceleration fields of both the solid and liquid phases simultaneously. Measurements were taken at turbulent conditions of a vertical pipe flow using glass spheres as the solid phase and fluorescent particles to indicate fluid phase motion. Nd-YAG pulse lasers acted as illumination sources and images were recorded by two monochrome CCD cameras. The two-color aspect of the technique was realized by placing optical filters in front of the cameras to discriminate between the phases. Cross-correlations and auto-correlations were applied to determine velocity and acceleration fields of the two phases. Results showing some of the capabilities of the technique as applied to a two-phase pipe flow experiment are provided. For the condition studied, it was found that there was turbulence suppression due to the solid phase and that the statistics associated with the acceleration probability distribution were different for the solid and fluid phases.

Momentum and energy transfer during phase change of water under high hydrostatic pressure

Abstract This work is aimed at investigating phase change processes of water under high pressure by help of an in situ visualization technique. High Pressure Digital Particle Image Velocimetry and Thermography (HP-DPIV and HP-DPIT) techniques are applied to a phase change process for the first time and temperature-convective fields are analysed. Peculiar local flow characteristics are observed, such as stagnation or extensional flow regions, for different processes. Pressure-induced thawing process demonstrates inhomogeneities in temperature field, whereas pressure-assisted thawing has almost a homogeneous temperature distribution. Moreover, one of the ice types which has a higher density than that of water (Ice III) is distinguished with an opposite direction of motion of the solid body, regarding Ice I, and it demonstrates different flow characteristics. Additionally, the processes are represented with conservation balance for mass, energy and momentum to render a global interpretation and to analyse process mechanisms. This work was partially presented in the High Pressure Bioscience and Biotechnology conference in Rio de Janeiro in October 2004. Industrial relevance: In this study, phase change processes of water under high pressure were investigated. The goal was to analyze the fluid-mechanical characteristics of the system and their interactions thermodynamically. A dimensional analysis was carried out. This tool helps in scale up analysis to reduce the number of necessary parameter variations to a minimum. Further, this study aims at a more precise understanding of the primary heat transfer mechanisms which represent an important aspect concerning uniform processing.

Digital particle image velocimetry (DPIV) technique in measurements of granular material flows, Part 2 of 3-converging hoppers

The flow evolution of an amaranth seed is being investigated in a wedge-shaped model made of Plexiglas. The objective of this paper is to recognise flow patterns in the flowing material, and also to depict evolution of velocity fields, flow field discontinuities, velocity profiles for cross-sections of the model, shear zones and flow streamlines using the digital particle image velocimetry (DPIV) optical technique. It is demonstrated that the DPIV technique used in the experiments enables quantitative analysis of the flow zones geometry. The technique also allows to reveal boundaries between flowing and stagnant zones and to extract velocity profiles at any selected sections of the model.